The present invention relates generally to the field of telecommunications. More particularly, the present invention relates to a method and apparatus for establishing sessions for Third Generation (3G) digital communications. Merely by way of example, the invention has been applied to a performing accelerated session setup for terminals communicating through a gateway, but it would be recognized that the invention may also include other applications.
Third Generation mobile networks allow their users access to a rich complement of multimedia services including audio, video, and data. The Third Generation Partnership Project (3GPP) is an industry consortium formed to advance the technology and acceptance of 3G mobile networks. The 3GPP has defined the 3G-324M Technical Specification that defines how terminals and the network interoperate in order to provide advanced services. Additionally, the Third Generation Partnership Project 2 (3GPP2) has adopted the 3G-324M Technical Specification.
The 3G-324M Technical Specification is based on the ITU-T (International Telecommunication Union, Standardization Section) H.324 Recommendation, that is, 3G-324M can be seen as a specific configuration of the H.324 Recommendation of the ITU-T. Other H.324-like terminals exist, such as H.324M terminals.
The 3GPP 3G-324M recommendations use and extend H.324 as follows:                1. The use of the ITU-T H.324 umbrella recommendation and its Annex C. This defines the overall videotelephony service, including H.223 and H.245.        2. The use of Annexes A and B of H.223 ITU-T.        3. The use of the mobile messaging facilities of H.245.        4. The use of specific audio and video codecs. For example, the GSM-AMR audio codec and the H.263 video codec are recommended. Other audio and video codecs are proposed as options.        
The 3GPP has defined a phased network evolution and has defined specifications for “Release 99”, “Release 5,” and “Release 6” networks in a logical network migration. Most mobile networks today use circuit switched interfaces and protocols (e.g., ISDN, ISUP, and TDM DSOs) in order to connect to fixed network telephony subscribers.
In a 3G-324M environment, interaction between terminal endpoints and the intervening network can be classified into three areas: call signaling, session signaling, and media exchange.
Call signaling is used to set up the bearer channel between endpoints. In 3G-324M, the bearer channel is typically a 64 Kb/sec channel.
Session signaling is used to define the framing used on the bearer channel, to negotiate media options, to create, identify, and control the operation of “logical channels” (which carry the media) within the multiplexed frames on the bearer, and to communicate control information between endpoints (such as the carriage of user key-presses).
3 G operators and service providers may offer their videotelephony subscribers equipped with 3G-324M terminals access to enhanced services (such as videoconferencing and videomail). They may also offer the subscribers the option of reaching users on other networks (such as the public internet or corporate, private, or another company's packet networks) and to establish with them videotelephony and conferencing sessions. In order to offer such services, the operators and services providers need to equip their networks with gateways that can provide protocol translation between the 3G terminals (e.g., 3G-324M) and the protocols of the services and/or users in the other networks. For example one protocol for multimedia communication that is used on the packet networks (e.g., public internet or corporate packet networks) is the ITU-T H.323 protocol. Another example protocol is the IETF (Internet Engineering Task Force) Session Initiation Protocol. Both H.323 and SIP are widely used as protocols for user or service connectivity in packet networks. There are variants on H.323 and SIP, that we call H.323-like and SIP-like, respectively.
The translation of protocols between 3G-324M terminals and H.323 or SIP terminals or services is typically done by a gateway function. The gateway converts the protocols including signaling, session establishment, media, as well as transport between circuit and packets.
During the 3G-324M and H.323 session setup phase, Session Signaling is used by both endpoints to advertise their Terminal Capabilities, to arbitrate for Master or Slave (which determines other protocol behaviors later), to add individual Multiplexer Table Entries and to open Logical Channels. In a videotelephony application, two Logical Channels are typically opened in each direction: one for audio data and one for video data. Note that a Session Setup sequence in this case requires six or eight round trip messages.
Once a session is set up, Session Signaling is used by 3G-324M and H.323 endpoints to communicate out of band control information, such as the transport of DTMF digits (by the “User Input Indication” message).
In the case of SIP, the number of round trip messages to perform the same functions is reduced, albeit at the loss of some flexibility. Although most SIP terminals use a different technique for transporting DTMF digits (they are sent inband in the media stream), SIP has the notion of transporting out of band control information (by the INFO method).
Importantly, for Session Signaling protocols commonly used by multimedia devices today (e.g., 3G-324M, H.323, and SIP), there are two important parts of the session setup for media: media negotiation and a media establishment.
An implementation of GSS functionality is denoted a “Full Proxy GSS,” if it faithfully and immediately relays requests and responses from one terminal to the other.
A Full Proxy GSS is straightforward to implement, as there is often a trivial mapping between messages from one network to messages in the other. As an example, both 3G-324M/H.324 and H.323 specify the use of the H.245 protocol at the session signaling layer.
A Full Proxy GSS converts messages received by one call half into an equivalent message (of the destination terminal protocol) for the other call half, with typically only some necessary modifications. As an example, if a request is made by an endpoint to open or close a logical channel, this request is immediately sent to the other endpoint: in effect, the Full Proxy GSS acts largely as an intelligent message translator/forwarder, and its operations are driven by the actions of the endpoints.
FIG. 1 is a simplified diagram illustrating a conventional call setup procedure between an H.324 device and an H.323 device using a proxy gateway. As illustrated in FIG. 1, call setup through a proxying gateway involves propagation of messages through the gateway from one terminal to the other with the minimal modifications needed for compatibility. The acknowledged messages incur a roundtrip delay on both sides of the gateway leading to a substantial slow down of the session setup for both terminals as compared to the time it would take for a session to be established to a like terminal.
Further, the session setup including media is further delayed by the propagation where the media is not transmitted from the H.323 device until after receipt of the OLCs Ack from the H.3245 device via the proxying gateway. Similarly media is not transmitted from the H.324 device until after receipt of the OLCs Ack from the H.323 device via the proxying gateway. As a result, a caller experiences long delays between the initial call setup messages and the delivery of media.
As mentioned with regard to the Full Proxy GSS and observed in FIG. 1, there are many similarities between the H.324 and H.323 protocols, in particular their use of the H.245 control protocol. For this reason, the two protocols are sometimes referred to as H.32X protocols.
Interworking between 3G-324M and a SIP network is somewhat more elaborate than interworking between 3G-324M and H.323, as SIP and RTSP use SDP instead of H.245 to communicate media options. There are several different flavors of SDP, so for purposes of simplicity, we may refer to them as SDP-like. However, the necessary mappings are readily available.
The apparent simplicity of the Full Proxy GSS comes with a cost. In order to accurately track the session state, the Full Proxy GSS must in some cases have detailed knowledge of the concrete protocols used by the call halves, and must often duplicate state and logic that is present in the concrete protocol implementations. In addition, since the Full Proxy GSS must track the session state of the underlying protocols, the state machines in the Full Proxy GSS may often be as complicated as the state machines of the underlying concrete protocols. Also, the Full Proxy GSS is substantially slower to set up a call than would be the case for either of devices attached if they were directly connected to peers in the same network, due to the propagation of messages and acknowledgements through both networks.
Thus, there is a need in the art for improved methods and systems for accelerated call setup in telecommunications applications through gateways.